New wood-based products have been developed alongside with traditional products of the forest industry. For example, different wood composite products in which wood-based materials are combined with plastic-based materials are known. Wood composites can be used for example in the building industry and in furniture applications. It has been known to manufacture wood composite products mainly by extrusion or injection molding.
In addition, different methods and apparatuses are known in the field of forest industry to manufacture a web, e.g. to manufacture a paper web or a chemical pulp web, and to dry it. It is known to form the web e.g. by wet web formation in which raw materials are formed into a liquid suspension which is led as an even layer from a headbox or the equivalent to a web forming section, e.g. a wire. Thereafter, water is removed from the web e.g. by pressing and/or drying by heat.
Known wood or natural fiber-based plastic mixtures have been manufactured by melt-mixing them together, typically utilizing the extrusion method. In these processes, plastic, dry wood or natural fiber material and compatibility-improving agents are mixed together at a high temperature in which the plastic materials are in the molten state. However, the dry wood and natural fiber materials, when dry, are strongly hydrogen bonded, and these bonds cannot be opened in the process. Consequently, the effect of the compatibilizing agents remains low, and in further processing the products have a strong tendency to reagglomerate.
Reference GB 1597369 discloses the preparation of a composite material from cellulose fibers and a polymer material in an aqueous suspension in the presence of a synthetic polymeric polyelectrolyte flocculating agent. As many hydrogen bonds as possible are tried to be formed between cellulose fibers to strengthen the cellulose network. The inert character of plastic particles is considered an advantage, whereupon the plastic particles do not interfere with hydrogen bonds which are developing between cellulose fibers. The resultant material is dried.
Reference WO 2009/008822 discloses the preparation of a composite material by a wet method so that a mixture of a lignocellulosic material and a thermoplastic material is formed, the mixture is led on a wet web to form a composite material, and the composite material is hot-pressed. The components are not chemically bound together.
Problems in the manufacture of known wood composite products include thermal stress on the fibers, breaking of the fibers, and reagglomeration of the fibers during the manufacturing process or further processing, whereupon the properties of the composite product, such as the mechanical properties and strength properties, are deteriorated. In later researches in the field it has been detected that the strong formation of hydrogen bonds between cellulose fibers causes reagglomeration of the cellulose fibers in further processes, which causes in products fiber bundles which clearly weaken the mechanical properties of the end product. Other problems include the undesirable odor and limited color options of the products, especially in connection with products containing lignin or hemicellulose.
Another problem in the manufacture of known wood composite products has been the lower production capacity compared with the manufacture of pure plastic products.